This invention is in the field of sewing machines, more particularly, it pertains to bobbin thread run-out indicators for sewing machines.
Their are in the prior art many forms of bobbin thread run-out indicators. There are for example those devices having bobbins with special cavities into which the thread must be wound initially in order to provide a thread supply remaining after indication of impending depletion, or as part of a means for obtaining an indication of impending depletion. There are also those sewing machines in which bobbin thread run-out indication depends upon reflection of a light source on the empty bobbin hub to a light sensor. Another device as disclosed in the U.S. Pat. No. 3,599,586 of Newman discloses the use of a fibre optic to transfer light from a source to a convex lens which focus the light rays adjacent the hub of the bobbin, which upon depleted thread condition will allow the light rays to pass to another fiber optic for visual perception or to other light sensor.
A problem with these and other bobbin thread run-out indicators is that a more or less high degree of manufacturing accuracy is required in the manufacture of the component parts thereof, and that no capability exists for varying the amount of thread remaining on the bobbin which will initiate the low bobbin indication.